This invention relates to continuously variable, power transmission systems and, more particularly, it concerns an improved multi-range split power transmission system of the type in which a continuously variable transmission unit (CVT) is bicoupled with a four shaft compound planetary device in a manner to increase the system efficiency in a compact system configuration.
Two range split power transmission systems, which incorporate a CVT bicoupled to a compound four-shaft planetary splitter in order to increase the efficiency of the transmission system to a higher level than that of the CVT by itself, are well known in the art. Two of the shafts of the splitter are selectably connected to the output shaft of the system to effect a "Mode A" operation in which the output speed of the CVT is subtracted from the system input speed and a "Mode B" operation in which CVT output speed is added to system input speed. In these transmission systems the CVT speed ratio is adjusted between two extreme values, the shifting from one range to the other occurring at generally synchronous speeds. The two selectable planetary shafts can be connected alternatively to the system input shaft (U.S. Pat. No. 4,363,247, 3,714,845 and UK Pat. No. 2,115,091), or to the system output shaft (U.S. Pat. Nos. 4,434,681, 4,382,392), or to the output shaft of the CVT (U.S. Pat. Nos. 3,969,958, 3,626,787, 3,675,507), directly or through a fixed ratio gear reduction.
In U.S. Pat. No. 4,856,371 issued to the present inventor on Aug. 15, 1989, a traction drive CVT is combined with a planetary gear power splitter in a system by which Mode A and Mode B operation of the system are effected in a manner to optimize CVT efficiency in a very compact system geometry. The compactness of the system is obtained in substantial measure by mounting the carrier of a double planet gear set in a tubular system input shaft which provides also the input of the CVT. The CVT output shaft extends within the tubular system input shaft to sun gears in mesh with the planets of the double planet set and is alternately engaged with the sun gears to attain the broad range of system input/output speed ratios.
It is known that the lower the speed range for each power splitting range, the higher the transmission system efficiency will be for a given CVT speed ratio range. It is therefore common to add a two range gear box to a two range split power transmission system in order to decrease the range of each operating Mode while keeping the same overall system speed ratio range. The problem with these arrangements is that the shifting between the middle ranges of the system, although performed at synchronous speeds, requires the operation of as many as four clutches simultaneously. This clutch operation, in turn, requires a complex control system and clutches capable of slipping under full torque causing dissipation of large amounts of heat. Therefore, four range split power transmission systems which require only synchronous shifting of two clutches between each range have been proposed. In these transmission systems, the two selectable shafts of the compound planetary splitter can be connected, each through a separate two-speed gear box to either the input shaft of the transmission system (U.S. Pat. No. 4,682,511), or the output shaft of the transmission system (Prof. F. Jarchow, VDI, University of Bochum), or the output shaft of the CVT. But these transmission systems have multiple parallel shafts as well as a total of four different pairs of gear sets which are coupled alternatively to two shafts of the compound planetary device, thus making such systems complex.
It is apparent, therefore, that there is a need for improvement in multi-range continuously variable power split transmission system of the type in which a CVT is bicoupled with a four shaft planetary splitter.